outside of corusant
*Chavo' ship exits outside Corusant.*
Chavo: Been some time, better check out what's going on.
*Chavo takes out his comlink and calls Greer
Ackbar Memorial Hospital, floor 304
*Greer takes out his ringing comlink*
Chavo: Yep, what's the news?
Greer: There's a rebellion going on. They might be right, some interesting assasinations are going on. Most of the good leaders are being killed off. And the senate almost nuked Corusant.
Chavo: What happened?
*Greer smiled, remembering the vampires, K'Warra and Gilan*
Greer: You wouldn't belive me if I told you. Gonna make a hell of a bedtime story for my grandkids though.
Chavo: What about the Empire, know where Cracken is?
Greer: No, though there's rumors about the Ulura Shardes.
Chavo: How accurate?
Greer: about a 7
Chavo: Intresting. I'm gonna follow up on it.
Greer: And my job offer?
Chavo: I'll think about it.
*Chavo entered a course for the shardes, and went into hyperspace again*
Vader's wife was very pregnant when she died 19 years ago. All of a sudden a 19 year old who is very strong in the force and has a distinct resembalance to him in his younger days rolls in from Vader's home planet with his old Master Obi-Wan (Who was the ONLY other person preset at the time his wife died. And to boot, the kid's last name is Skywalker.
So in answer to your question, he knew Luke was his son because his name is Darth Vader, and not Darth Retard.
-Forum post on why Vader knew Luke was his son.